1. Field of the Invention
The invention relates to a leakage diagnosis device and leakage diagnosis method that carry out a leakage diagnosis on the basis of an internal pressure state of an airtight compartment, formed in an evaporative fuel processing mechanism, including a fuel tank, for an internal combustion engine, by emitting gas from the compartment via a canister during a stop of the internal combustion engine.
2. Description of Related Art
There is known a technique for diagnosing whether there is a leakage from an evaporative fuel processing mechanism, including a fuel tank, for an internal combustion engine, on the basis of an internal pressure variation at the time when the inside of the evaporative fuel processing mechanism is decreased in pressure by a pump during a stop of the internal combustion engine. When the pressure is decreased by the pump in order to carry out such a leakage diagnosis, it is required to prevent leakage of fuel vapor from a canister to an outside.
In order to prevent such a leakage of fuel vapor, there is proposed a technique for carrying out leakage diagnosis only when fuel vapor adsorbed in the canister during refueling is sufficiently released into intake air through purging during operation of the internal combustion engine thereafter (see page 8 to page 10 and FIG. 2 of Japanese Patent Application Publication No. 2010-216287 (JP 2010-216287 A)).
Particularly, in an internal combustion engine mounted on a hybrid vehicle, the internal combustion engine frequently stops, so purging itself is also repeatedly carried out and stopped in a short period of time. Therefore, detecting a purge amount in a single operation of the internal combustion engine does not determine whether fuel vapor adsorbed in the canister has been sufficiently purged.
In order to take measures for this inconvenience, in JP 2010-216287 A, in the internal combustion engine that is repeatedly operated and stopped, a purge amount in each operation is accumulated, a fuel adsorption state of the canister is determined on the basis of the accumulated purge amount and a leakage diagnosis is carried out in a state where fuel vapor has been sufficiently removed from the canister.
In a hybrid vehicle, particularly, a plug-in hybrid vehicle, when the hybrid vehicle is charged from an external power supply, the internal combustion engine may be stopped for a long period of time even when vehicle frequently repeats travelling. In such a case, in order to early find a leakage, it is required to carry out a leakage diagnosis not only once after a stop of the internal combustion engine but repeatedly.
When a leakage diagnosis is carried out, the pressure inside the evaporative fuel processing mechanism, including the fuel tank, is decreased by emitting gas from the evaporative fuel processing mechanism to an outside via the canister with the use of the pump, so fuel vapor in the fuel tank flows into the canister each time a leakage diagnosis is carried out.
In JP 2010-216287 A, in the first leakage diagnosis immediately after a stop of the internal combustion engine, it is possible to determine whether to carry out a leakage diagnosis on the basis of the accumulated purge amount (a purge fuel concentration used to calculate the accumulated purge amount) detected during operation of the internal combustion engine. Therefore, even when a leakage diagnosis is carried out, fuel vapor is not likely to leak from the canister to an outside.
However, when a leakage diagnosis is repeated thereafter in a state where the internal combustion engine is stopped, fuel vapor is repeatedly introduced from the fuel tank into the canister. Thus, the amount of :fuel adsorbed in the canister gradually increases, and a remaining adsorption capacity decreases. Thus, when a leakage diagnosis is repeated in a state where the internal combustion engine is stopped, the canister finally becomes a saturated state, so there is a possibility that an erroneous diagnosis is carried out or fuel vapor leaks from the canister to an outside.
Therefore, when a leakage diagnosis is carried out only once immediately after a stop of the internal combustion engine and the stop of the internal combustion engine extends over a long period of time, the frequency of leakage diagnosis may be insufficient.